1. Field of the Invention
The present invention relates to a droplet deposition position error measurement method, a droplet deposition position error adjustment method, a droplet ejection control method and an image forming apparatus, and more particularly, to a droplet deposition position error adjustment method suitable to an inkjet recording apparatus which forms an image by ejecting droplets of ink onto a recording medium from a print head in which a plurality of ink ejection ports (nozzles) are formed, and an adjustment method and a droplet ejection control method for correcting such an error, and an image forming apparatus using this droplet ejection control method.
2. Description of the Related Art
In an inkjet recording apparatus (an inkjet printer) comprising a print head having a plurality of nozzles, if the print head is installed at angle of inclination with respect to the prescribed installation position (the ideal installation position according to the design), then dots are not formed at the desired positions, and hence there is a problem in that the recording positions (droplet deposition (landing) positions) of the dots are displaced.
In view of the aforementioned problem, Japanese Patent Application Publication No. 11-277721 discloses a method in which, in order to determine the degree to which a print head is inclined with respect to the recording paper conveyance direction, a plurality of line patterns of a length in which a plurality of ink ejection ports are aligned are printed, the print results of this plurality of line patterns are determined visually, and the installation position of the print head is adjusted.
Furthermore, Japanese Patent Application Publication No. 2001-129983 discloses a structure of a carriage section which is capable of adjusting the inclination of the head, having a composition in which the inclination of the head is adjusted on the basis of a correlation between the results of the test recording and a previously determined inclination correction amount.
Japanese Patent Application Publication No. 11-277721 and Japanese Patent Application Publication No. 2001-129983 both have technical contents based on the serial scanning type of inkjet recording apparatus which records images while reciprocally moving a print head in a direction perpendicular to the direction of conveyance of the recording paper (the sub-scanning direction).
The droplet deposition position error caused by the inclination of the recording head was studied in more detail, and then in addition to the problems disclosed in Japanese Patent Application Publication No. 11-277721 and Japanese Patent Application Publication No. 2001-129983-(namely, jaggedness of the printed lines due to displacement of the dot positions between scans in the serial scanning method), there are also problems of the following kind, especially in a case where the nozzles are arranged at high density.
(1) Banding (Unevenness) Occurring at the Return Positions in a Matrix Head
In an inkjet recording apparatus, in order to print images of high quality at high speed, a recording head having a plurality of nozzles arranged in a two-dimensional matrix (so-called “matrix head”) have been proposed. If droplets are ejected from a matrix head of this kind, and if the head is installed in an inclined fashion, then the pitch between the nozzles from which droplets are ejected to form mutually adjacent dots on the recording medium varies, and banding may occur at the regions corresponding to the return positions of the matrix configuration (hereinafter, this banding is also called “matrix return position banding”). This phenomenon is illustrated in FIG. 13.
FIG. 13 is a plan view perspective diagram showing a schematic view of a portion of a matrix head. The left-hand diagram in FIG. 13 shows a case where a matrix head is installed correctly, and the right-hand diagram in FIG. 13 shows a case where the matrix head is installed in a position rotated in the counter-clockwise direction, in the plane of the paper. In FIG. 13, reference numeral 210 denotes a nozzle and reference numeral 212 denotes a pressure chamber corresponding to each nozzle.
Each pressure chamber 212 corresponding to a nozzle 210 is coupled to a common flow channel for ink supply (not shown) via an independent supply port (not shown), in such a manner that ink is filled into each pressure chamber 212 from the common flow channel. Furthermore, pressure generating elements (for example, piezoelectric elements) (not shown) are provided corresponding to the pressure chambers 212, and hence ink droplets can be ejected from the nozzles 210 by controlling the driving of the pressure generating elements in accordance with the print data. By controlling the ink ejection timings of the nozzles 210 while the recording medium is conveyed, it is possible to record a desired image on the recording medium.
As shown in the left-hand diagram in FIG. 13, if the matrix head is installed correctly, then the dots recorded onto the recording medium are arranged at equidistant intervals in the main scanning direction. On the other hand, as shown in the right-hand diagram in FIG. 13, if the matrix head has been installed at a position rotated in the count-clockwise direction, in the plane of the paper, then the dot interval corresponding to each return position of the matrix arrangement widens markedly compared to the other positions (see B1 in FIG. 13). Therefore, banding occurs with respect to each of the return positions in the matrix arrangement. In the description given below, a group of nozzles corresponding to each return position in the matrix arrangement (a nozzle group as indicated by C in FIG. 13) is called a “nozzle block”.
(2) Banding Occurring at Joint Sections Between Short Heads
In cases where a line head is constituted by joining together a plurality of short head modules (short heads), if the positions of the joints between the short heads are displaced, then the pitches between the nozzles from which droplets are ejected to form mutually adjacent dots vary, and banding occurs at the regions corresponding to the joint sections between the short heads (below, this type of banding is also called “short head joint banding”). This phenomenon is illustrated in FIGS. 14A and 14B.
FIG. 14A shows a case where the short heads 221 and 222 are installed correctly, and the dots recorded onto the recording medium are arranged at equidistant intervals in the main scanning direction. On the other hand, FIG. 14B shows a case where the short heads 221 and 222 are installed in positions where they are distanced from each other (namely, the right-hand short head is shifted in the rightward direction from the normal installation position, in the diagram). In this case, the dot interval at a joint section between the short heads is widened markedly in comparison with the other dot intervals (see B2 in FIG. 14B). Consequently, banding occurs at the junction between the short heads.
Next, the reasons why the dots are caused to be displaced from their desired positions are explained. The causes of displacement of the dot positions include errors that are intrinsic to the nozzles, in addition to an error caused by incorrect installation of the head as described above. Intrinsic nozzle errors are caused by soiling in the vicinity of the nozzle (a non-uniform lyophobic state), or manufacturing errors in the nozzle section, or the like, and consequently the liquid droplets ejected from the nozzles are displaced from the desired ejection direction (which is normally, the perpendicular direction with respect to the nozzle surface). Usually, these errors are thought to occur independently at each nozzle (in other words, it is thought that there is no correlation between nozzles).
Intrinsic nozzle errors may also be involve an error which does not create a shift in the ejection direction (for example, an error caused by a nozzle hole being formed in a place that is displaced from the desired position, in the manufacturing stage); however, with current manufacturing technology, this hardly ever happens, and therefore an intrinsic nozzle error is principally due to an error involving displacement of the ejection direction. Therefore, in the following description, reference to an intrinsic nozzle error is taken to be an error in terms of displacement of the ejection direction of a nozzle itself.
According to Japanese Patent Application Publication No. 11-277721 and Japanese Patent Application Publication No. 2001-129983 described above, the amount of displacement in the installation of the head is estimated by actually performing a test print; however, in these methods, it is difficult to distinguish between an error caused by incorrect installation of the head, and an error that is intrinsic to the nozzles. Hence these methods are not suitable for a case where a head position is sought to be adjusted precisely according to a test pattern.
In particular, in order to prevent “matrix return position banding” or “short head joint banding”, if the position of the head is adjusted according to a test pattern, then the amount of displacement of the dot positions that are to be measured is of the order of several μm, and this is the same order as that of the positional error that is intrinsic to the nozzles. Therefore, it is difficult to evaluate the amount of adjustment (the amount of correction) of the head position accurately, on the basis of the measured values of the amount of displacement of the dot positions.